Plato’s The Republic (Book Vll / 514a – 515c).
“Next then,” I said, “make an image of our nature in its education and want of education, likening it to a condition of the following kind. See human beings as though they were in an underground cave-like dwelling with its entrance, a long one, open to the light across the whole width of the cave. They are in it from childhood with their legs and necks in bonds so that they are fixed, seeing only in front of them, unable because of the bond to turn their heads all the way around. Their light is from a fire burning far above and behind them. Between the fire and the prisoners there is a road above, along which see a wall, built like the partitions puppet-handlers set in front of the human beings and over which they show their puppets”
“I see,” he said.
“Then also see along this wall human beings carrying all sorts of artifacts, which project above the wall, and statues of men and other animals wrought from stone, wood, and every kind of material; as is to be expected, some of the carriers utter sounds while others are silent.”
“It’s a strange image,” he said, “and strange prisoners you’re telling of.”
“They’re like us,” I said. “For in the first place, do you suppose such men would have seen anything of themselves and one another other than the shadows cast by the fire on the side of the cave facing them?”
“How could they,” he said, “if they had been compelled to keep their heads motionless throughout life?”
“And what about the things that are carried by? Isn’t it the same with them?”
“Of course.”
“If they were able to discuss things with one another, don’t you believe they would hold that they are naming these things going by before them that they see?”
“Necessarily.”
“And what if the prison also had an echo from the side facing them? Whenever one of the men passing by happens to utter a sound, do you suppose they would believe that anything other than the passing shadow was uttering the sound?”
“No, by Zeus,” he said. “I don’t.”
“Then most certainly,” I said, “such men would hold that the truth is nothing other than the shadows of artificial things.”
-Plato The Republic (translated by Allan Bloom)